1. Field of the Invention
The present invention relates to a lens barrel which holds a zoom lens consisting of multiple lens groups as well as to an image taking apparatus which is provided with the lens barrel and takes images by capturing light from a subject entering through the zoom lens held by the lens barrel.
2. Description of the Related Art
Today, digital cameras are in rapidly expanding use, and they are required to provide ever higher image quality along with reductions in overall size and profile thickness. As a digital camera to meet these requirements, there is proposed a thin visiting card-size digital camera equipped with a lens barrel holding a zoom lens consisting of multiple lens groups (see non-patent literature 1, Internet URL http://www.business-ijp/sentan/jusyou/2003/pentax/) and is already commercially available. This digital camera permits zooming with high image quality with an optical zoom while earlier thin digital cameras use an electronic zooming function to achieve a high degree of magnification.
One of the notable needs of users is equipment with such an optical zoom capable of zooming at an even higher magnification while realizing further reductions in size and profile thickness.
Patent Document 1 (Japanese Patent Laid-Open No. 2003-295031) proposes a technique for collapsing a lens barrel equipped with a zoom lens into a thin camera body by contriving an improved way of forming cam grooves.
The internal configuration of the lens barrel disclosed in this patent literature 1 is described below.
FIGS. 1, 2 and 3 are sections taken along the optical axis of a lens barrel built into a digital camera. Of these FIGS. 1 through 3, FIGS. 1 and 2 show the lens barrel in its extended state. Incidentally, patent literature 1 states that FIG. 1 shows a telephoto end and FIG. 2, a wide-angle end. FIG. 3 shows the lens barrel in its collapsed state. Further, FIG. 4 is a developed view illustrating cam grooves which function when the lens barrel is extended out or collapses into the camera body.
The configuration of this lens barrel 100 is described below with reference to FIG. 1 through FIG. 4.
This lens barrel 100 holds a zoom lens of a four-group configuration consisting of a first lens group 210, a second lens group 220, a third lens group 230 and a fourth lens group 240. By moving the second lens group 220, out of these four lens groups, in the direction of the optical axis, the focal distance is adjusted, and moving the fourth lens group 240, serving as the focus lens, in the direction of the optical axis, focusing is accomplished.
The first lens group 210 is held in an inner tube 110. This inner tube 110 is provided with cam pins 111, and these cam pins 111 engaged with cam grooves 121 formed in the inner surface of an outer tube 120 (see FIG. 4). Three such cam pins 111 are disposed at unequal intervals on the outer wall of the inner tube 110, and three cam grooves 121 to engage with those cam pins 111 are formed at unequal intervals in the inner wall of the outer tube 120 (see FIG. 4). Therefore, when the rotation of a zoom motor 270 is transmitted from a coupling gear 271 (see FIG. 2) to a gear 124 disposed on the inner wall of the outer tube 120 to turn the outer tube 120, the first lens group 210 extends together with the inner tube 110 along the shape of the cam grooves 121.
The second lens group 220 is held by a lens group holding frame 221, and three cam pins 222 are disposed at unequal intervals on the circumference of that lens group holding frame 221. Those cam pins 222 engage with respective cam grooves 122 formed in the inner wall of the outer tube 120 (see FIG. 4). A guide rod 1132 is passed through a through-hole 221a formed in the lens group holding frame 221 of the second lens group 220. When the outer tube 120 turns, the second lens group 220, guided by that guide rod 1132, moves in the direction of the optical axis. The guide rod 1132 is held by a tip support 1132a, and a guide rod 1133 to guide the movement of the third lens group 230 is supported by another tip support 1133a. These tip supports 1132a and 1133a, besides respectively supporting the two guide rods 1132 and 1133, are also used as members to support the inner tube 110. These tip supports 1132a and 1133a are equipped with an intermediate frame 1101 and a retainer ring 1102. The intermediate frame 1101 is inserted slidably along the inner wall of the inner tube 110, while the retainer ring 1102 is provided at the rear end of that inner tube 110 to prevent the intermediate frame 1101 from moving backward. The intermediate frame 1101 and the retainer ring 1102 are respectively equipped with spring pegs 1101a and 1102b. When the intermediate frame 1101 is extended together with the inner tube 110, a spring 1103 bridging those spring pegs urges the intermediate frame 1101 forward along the sliding surface to restrict the movement of the intermediate frame 1101 not to let the intermediate frame 1101 move backward.
Further, one cam groove 123 (see FIG. 4) to engage with cam pins (not shown) disposed on a lens group holding frame 130 of the third lens group 230 is formed in the outer tube 120 between the cam grooves 121 and 122. Therefore, when the outer tube 120 turns, receiving the driving force of the zoom motor 270 via the coupling gear 271 and the gear 124 (see FIG. 2), the third lens group 230 also moves in the direction of the optical axis along the shape of the cam groove 123. A shutter unit 131 is linked to the lens group holding frame 130 holding this third lens group 230.
A through-hole 131a is formed in this lens group holding frame 130, too, as in the lens group holding frame 221, and the guide rod 1132 is passed through this through-hole 131a. Further, the guide rod 1132 is also passed through a through-hole 141a formed in a lens group holding frame 140, to be described after wards, for holding the fourth lens group 240. In this way, the example taken up here has a configuration in which the second lens group 220, the third lens group 230 and the fourth lens group 240 are guided in movement by the common guide rod 1132 to prevent the optical axis from misalignment.
The extending operation of the lens barrel 100 having such a structure is described in detail with reference to FIG. 4.
When the outer tube 120 rotates driven by the zoom motor 270, the inner tube 110 is extended from a collapsed state along the shape of the cam grooves 121 (the area marked with symbol A) to its extended position (the end toward symbol B of the area marked with symbol A), and held in that extended position (the area marked with symbol B). Until the outer tube 120 is held in this extended position, the second lens group 220 moves in the area marked with symbol C along the shape of the cam grooves 122, and reaches the end of the area marked with symbol C when the inner tube 110 has been extended to its extended position. When a zoom switch (not shown) is operated then, the second lens group 220 enters into the area marked with symbol D according to the operation of that zoom switch. If the zoom switch continues to be operated, it will reach the end of the symbol D area. On the other hand, when the outer tube 120 is turned, the third lens group 230 moves from its collapsed position in the extending area (the area marked with symbol E) along the cam groove 123 to reach its extended position (the intersection between symbol E and symbol F) and, even if the zoom switch is operated, remains held in that extended position (the area marked with symbol F).
By arranging the cam grooves effectively in the inner wall of the outer tube 120 in this way, it is made possible to move the first lens group 210, the second lens group 220 and the third lens group 230 out of the four lens groups in the direction of the optical axis along the shapes of the cam grooves by the rotation of the single tube 120, and to perform zooming by moving the second lens group in the direction of the optical axis according to the operation of the zoom switch.
In the lens barrel 100 shown in FIGS. 1 through 3, the aftermost fourth lens group 240 out of the four lens groups constituting the zoom lens is used as the focus lens. Since this zoom lens held by the lens barrel 100 has a high zoom ratio, the traveling distance of the fourth lens group 240 to function as its focus lens should be relatively long. Therefore in this example, a column screw 1131 (see FIG. 1) having a length equal to that long traveling distance is disposed along the optical axis, a nut 141b is fixed to the lens group holding frame 140 holding the fourth lens group 240, and the column screw 1131 is screwed into that nut 141b to enable focusing to be accurately accomplished.
The rotational driving force of a focus motor (not shown) is transmitted to this column screw 1131 via a gear train (not shown) to rotate this column screw 1131 and, along with that rotation, a lens group holding frame 141 guided by the column screw 1131 and the guide rods 1132 and 1133 moves in the direction of the optical axis correspondingly to the rotation of the column screw 1131 to perform focusing. When an image taking lens begins catching a subject, image data are generated by an image pickup device 280, the focus position is detected on the basis of the image data, and focusing is performed by turning the column screw to move the fourth lens group 240, which serves as the focus lens, to that detected focus position.
If a shutter button (not shown) is pressed after focusing is accomplished, the shutter unit 131 arranged on the lens group holding frame 130 holding the third lens group 230 is driven in synchronism with full pressing of the shutter button to take an image. Light from the subject having passed the first lens group 210, the second lens group 220, the third lens group 230 and the fourth lens group (focus lens) 240 forms an image on the light-receiving surface of the image pickup device 280, and this image pickup device 280 generates an image signal which represents the image of the subject formed on that light-receiving surface.
By arranging the multiple cam grooves effectively in the inner wall of the outer tube in this way, bringing them into cam engagement with the inner tube, and causing the aftermost lens group of the zoom lens to function as the focus lens, the length of the lens barrel in the collapsed state is reduced to relative compactness to be accommodable in a relatively thin body, and a lens barrel which can achieve a high zoom ratio when extended is thereby realized.
However, the configuration described above in which the travel of the second lens group 220, the third lens group 230 and the fourth lens group 240 is guided by the common guide rod 1132 extending over a great length from the light-receiving surface of the image pickup device imposes a limit to the thinning of the profile.
Apart from that, patent literature 2 (Japanese Patent Laid-Open No. 6-67076) discloses a three-group zoom lens barrel which can provide a long traveling distance for lenses at a small angle of rotation of a cam ring.
According to this patent literature 2, though there is no guide rod or the like extending over a great length from a base which fixes an image pickup device, the second and third groups out of the three lens groups, prevented from rotating by a straight-ahead key, move in the direction of the optical axis via a cam mechanism, and supported only via that cam mechanism. Accordingly, it is extremely difficult to adjust the optical axis with high precision and, if this technique is applied to a zoom lens of a four lens-group configuration, adjustment of the optical axis and keeping its accuracy will be extremely difficult, making it prohibitively difficult to extend this technique as it is to a four lens-group configuration.